Obese adolescents with insulin resistance are predisposed to the development of Type II diabetes mellitus in adulthood . This study examines the effects of Metformin on insulin sensitivity in obese adolescents with elevated fasting insulin levels and first degree relatives with Type II diabetes. The trial is double-blind and placebo controlled. Obese adolescents will undergo a rapid intravenous glucose tolerance test, permitting baseline assessment of insulin sensitivity using the Bergman minimal model. Patients will then be assigned to drug or placebo groups for a total of six months, with monitoring of fasting plasma glucose, insulin, glycosylated hemoglobin, plasma lipids and weight, blood pressure and total body fat. Insulin sensitivity will be assessed at the end of the trial by repeating the intravenous glucose tolerance test. The results of these studies may provide a new approach to the prevention of diabetes mellitus in patients predisposed to the disease.